The present invention relates to a method of manufacturing glass optical elements for molding with high surface precision lenses, for example, in which one or both surfaces are concave and have a ratio b/a of the peripheral thickness b to the center thickness a of at least 1.5.
Methods in which softened glass is pressed with a mold having the shape of the product or similar to that of the product to directly manufacture lenses without grinding or polishing (precision pressing methods) are widely utilized. Precision pressing is employed to manufacture the optical system lenses of a variety of optical equipment and products such as digital cameras and video cameras. In precision pressing, the molding surface of the mold must be transferred to the glass as precisely as possible.
However, in lenses in which one or both surfaces are concave, even though molding surfaces are transferred to the glass, the glass sometimes moves back in the direction of one of the surfaces while solidifying, or a transferred surface that has been molded deteriorates, sometimes precluding adequate surface precision. This tendency is marked in the molding of lenses with a ratio b/a of the peripheral thickness b to the center thickness a of at least 1.5.
Accordingly, an object of the present invention is to provide a method of manufacturing glass optical elements permitting the molding of lenses that are concave on one or both surfaces with high surface precision.
Research conducted by the present inventors resulted in the discovery that the above-stated problem was solved, in the case of a lens with one concave surface, by delaying cooling of the concave surface relative to cooling of the surface (flat or convex) on the opposite side, and in the case of lenses with two concave surfaces, by delaying cooling of the concave surface with the smaller radius of curvature relative to cooling of the concave surface on the opposite side with the larger radius of curvature.
The present invention relates to a method for manufacturing a glass optical element having at least one concave surface, including:
softening a glass molding material by heating,
molding the softened material with a first mold having a first molding surface and a second mold having a second molding surface by applying a pressure, the first molding surface includes a first concave forming surface, the second molding surface includes a convex forming surface, a planar forming surface or a second concave forming surface, the second concave forming surface having a curvature radius greater than that of the first concave forming surface,
whereby shapes of the first molding surface and the second molding surface are transferred to the material,
cooling the material so that a temperature of the material reaches a temperature equal to or lower than glass transition temperature (Tg), and
removing the cooled material from either of the first mold or the second mold,
where in the cooling step, a second temperature of the second mold reaches the glass transition temperature prior to a time when a first temperature of the first mold reaches the glass transition temperature.
In a preferred embodiment of the present invention, one of either the first mold or the second mold comprises an upper mold, and the other comprises a lower mold; and said first mold preferably comprises an upper mold and said second mold preferably comprises a lower mold; and when the second temperature reaches the glass transition temperature, the first temperature is preferably at least 5 degree centigrade higher than the glass transition temperature.
One of the embodiments of the present manufacturing method (Manufacturing Method 1) is that for glass optical elements, in which one optically functional surface is concave, and includes:
softening a glass molding material by heating
press-molding the heat-softened glass molding material with a forming mold including an upper mold and a lower mold with molding surfaces for forming the optically functional surfaces of the glass optical element to be molded, one of these molding surfaces being a concave forming surface and the other being a convex forming surface or a flat forming surface, thereby transferring shapes of the molding surfaces to the glass material being molded;
cooling the forming mold to cool the molded glass to a temperature equal to or lower than the glass transition temperature (Tg) of the glass; and
removing the cooled glass from the forming mold;
where the cooling is conducted such that the temperature ta2 of the mold with the convex forming or flat forming molding surface reaches Tg before the temperature ta1 of the mold with the concave forming molding surface.
The second embodiment of the present manufacturing method (Manufacturing Method 2) is that for glass optical elements in which both optically functional surfaces are concave, including:
softening a glass molding material by heating
press-molding the heat-softened glass molding material with a forming mold including an upper mold and a lower mold with molding surfaces for forming the optically functional surfaces of the glass optical element to be molded, both of these molding surfaces being concave forming surfaces. thereby transferring shapes of the molding surfaces to the glass material being molded;
cooling the forming mold to cool the molded glass to a temperature equal to or lower than the glass transition temperature (Tg) of the glass; and
removing the cooled glass from the forming mold;
where the cooling is conducted such that the temperature tb2 of the mold with molding surface having the larger radius of curvature reaches Tg before the temperature tb1 of the mold with the molding surface having the smaller radius of curvature.